How do we see edges?

There are two scales of coloured fountains, red-green and blue-green one. With rising number, the gray part turns stronger and the coloured part weaker.

Please always find the number where both gray and coloured part appear with the same intensity. One number for red-green and one for blue-green. If you see a whole range of equal numbers, please report the middle of the range.

These numbers seem to change from person to person and also with the time of the day. It may mean the world looks different to different people and also in the morning and in the evening.